making a rollbar safe and removable without a sawzall.

particularly this one

20171231_132007 by Michael Crawford, on Flickr

20171231_132029 by Michael Crawford, on Flickr

spec miata main hoop segment, 2000 miata if you needed to know.

this is a challenge car, but im building in maximum safety . its a thing i have....

anyway, i learned about the absolute pain in the dick of a fully welded, door bar equipped cage with the AMC. that car was never destined for anything but competition, so i dealt with it. made everything 100 times ore difficult after it went in though. always in the way. 

my daily has a hard dog ace in it. a bolt in. that still in the way for a lot of various service procedures. the challenge car will have essentially a radically overcomplicated hard dog sport. with extra tubes. as you can see in the picture (yes, no rear downtubes yet, as i haven't made them. they will go as far rearward as i feel comfortable with going and still being able to have some sort of top. im hoping to land them after the rear strut mounts.)

anyway, this thing needs feet made first and foremost. the hard dog has roughly 6 inch wide plates under the main hoop that go way down the rear bulkhead to clear a boxed section for the body, and bolt in there. they use some sore of backer plate on the other side. same way with the rear downtubes. plates under the body that are cut to clear the body. 

i have no qualms about cutting things in this car. or welding. but i want to be able to remove the bar for service fairly easily. 

my thought was to make a 1/8 steel plate that going into the hole where the seatbelt reel used to live and down the body about 10 inches. that would make it roughly a 12x6 solid plate. weld a few grade 8 1/2 nuts to this plate, and weld the plate to the body. make a corresponding plate welded to the main hoop feet with holes in it to put bolts through. in addition, weld a tab to the hoop that bolts to the seatbelt tower where the factory seatbelt guide mounted. i would also weld the seatbelt towers completely after cutting my clearance notches and such in to make them as strong as possible. 

rear downtube feet would be done the same. 

but i question if this is the right approach. all the bolt in bars have the factory floor sandwiched, with a bigger plate on the interior side, and a smaller on the underside, and then use a nylock nut and bolt through. im uncertain if this is a safety reason, or an install reason, or both. may even have a touch of manufacturing reasoning. 

what are the thoughts of you guys? is this a good approach? would it pass tech for SCCA autocross, NHRA 11.99 and quicker, and possible HPDE in the future?

too long didn't read: can i weld a 1/8 plate to the floor with captured nuts, make a similar plate welded to the roll bar, and bolt the thing in and pass tech/not die?

Go find an NHRA rulebook.  There is a section all about removable members and so on.  If it meets their spec, it will probably make it most places.

Ive been digging around prior to this post, and cant seem to find the actual rule book online. Just forum posts, all of which conflict with one another.  Especially when it comes to miatas. Or convertibles. 

www.nhraracer.com/content/general.asp?articleid=47000&zoneid=132

1. Cached
2. Similar

Junior Dragster Racing League Rulebook. 2017 to 2018 JDRL Rulebook Amendments · 2018 JDRL Rulebook. Hot Rod Heritage Racing Series Rulebook. 2016 Hot Rod Heritage Racing Series Rulebook Supplement. 2016 to 2018 NHRA Heritage Racing Series Rule Supplement Amendments. Exhibition Rulebook.

Maybe this will help.

If I'm reading the NHRA book right you need 6x6 .125 thick sandwich plates with at least 4 3/8 bolts, I'd guess they won't care if the lower plates and nuts are tacked in place. 

Relevant excerpts. 

Unibody cars with stock floor and firewall (wheeltubs permitted) may attach roll bar with 6-inch x 6-inch x .125-inch steel plates on top and bottom of floor bolted together with at least four 3/8-inch bolts and nuts,

On unibody cars with stock floor and firewall (wheel tubs permitted), the roll cage may be bolted or welded to the floor/rocker box via 6-inch x 6-inch x .125-inch steel plates similar to the roll-bar attachment requirements of paragraph 4:10 in this section

That bar has a lot going on.

In reply to Dirtydog :

thanks!!

bolt in states that plates must be minimum 6x6 with minimum 4 3/8 bolts on unibody cars. is said with bolt in plates that they had to sandwhich the floor. this is NHRA regs from your links. 

why? is there an engineering reason for this that im not seeing? ill do it purely because i have to, but why do i have to? 

there were more responses while i was reading the link and replying. 

there IS a lot going on with this bar. but, i like it a lot. especially how its bent for rearward seat travel and helmet clearance behind the driver. also, it looks badass, which is obviously a requirement for all safety gear 

the rear downtubes will also receive a full X in between the legs. just for completion of the "whole lotta tubing going on" look. 

i guess i could weld the plates on the underside of the car with the nuts welded to them. i honestly hadn't thought about that.....

probably be easiest to make the plates on the top side, and then trim whatever makes the most sense to weld them to the bottom. 

and seeing 3/8 bolt spec makes me question wether the 1/2 bolts would be beneficial, or so big they would sacrifice the integrity of the plates. 

The 6x6 gives a good footprint, the backing plate keeps it from pulling out of the floor in a side impact.

I always thought that the rollbar foot plates that sandwich the floor should be unequal in size relative to each other so as to avoid the cutting action that two identically sized plates would create.

Streetwiseguy said:

The 6x6 gives a good footprint, the backing plate keeps it from pulling out of the floor in a side impact.

That makes sense but how is it different with the plate on the underside vs welded to the top side? Or is it any different?

ebonyandivory said:

I always thought that the rollbar foot plates that sandwich the floor should be unequal in size relative to each other so as to avoid the cutting action that two identically sized plates would create.

Good reminder thank you

Dusterbd13 said:

Streetwiseguy said:

The 6x6 gives a good footprint, the backing plate keeps it from pulling out of the floor in a side impact.

That makes sense but how is it different with the plate on the underside vs welded to the top side? Or is it any different?

I am not a huge fan of boltins myself.   I prefer welding to the top of the hill/rear seat riser area.  Going to the floor pan always puts me too close to the steering wheel, and it doesn't make sense to me to mount a roll bar to a piece of 22 ga sheet metal spot welded to the sill.

Streetwiseguy: is it legal to bolt or weld a rollbar to the rise behind the driver?  The rules quoted say floor or rocker box.  The classic Hard Dog Miata rollbar bolts to that rise but I’m going to guess that a lot fewer Miata go drag racing than say to the autocross.  This is relevant to my Fiero Challenge interests so apologies to dusterbd13 for the threadjack.

Plate on top keeps the tube from punching through the floor, plate on the bottom keeps the bolts from ripping through the floor. 

More surface area effectively increases the amount of force needed to cause the floor to fail. 

ebonyandivory said:

I always thought that the rollbar foot plates that sandwich the floor should be unequal in size relative to each other so as to avoid the cutting action that two identically sized plates would create.

Not to be contrary, but can anyone back this up? 

If there's any truth to this, it would be great to know. I just don't see how having the plates mirror each other would create a cutting action. 

In reply to Crackers :

I don't see it. Only one of the plates is going to be putting shear stress (assuming we're worried about shear, since "cutting" usually implies shear) in to the floor at a time. Both being the same size shouldn't be an issue.

Only place I ever heard of the different size plates was w/ the 4X4/ off road crowd. Not by a sanction either, just for off road builds. They also radius the plate edges facing the floor. Never found out exactly why but I heard it a lot over the years, must have been an incident w/ plates shearing the floor. I built two bolt-in cages (XJ and in truck cab) and followed the recommendations, not a big deal. I still take it as rule of thumb.

Nowhere else though. The  SCCA legal Autopower bolt-in cage plates were the same dimension top and bottom. I'd think SCCA would address backup plate size if any sanction would. Dunno if NHRA even allows bolt-ins, always thought they had to be weld in.

.02

Dusterbd13 said:

Streetwiseguy said:

The 6x6 gives a good footprint, the backing plate keeps it from pulling out of the floor in a side impact.

That makes sense but how is it different with the plate on the underside vs welded to the top side? Or is it any different?

The sandwich is much stronger.

Picture this as a Drywall wall. You've got a plywood plate and a tube glued to the face of the drywall, and a 2nd one screwed through the drywall into another plywood plate. 

If you try to push the tube straight through the drywall, there is equal strength in both cases. This does not, however, adequately pattern what a roll bar is subjected to. 

Roll bars never try to push straight through. They are hit at every angle. 

So, if you try to pull the tube off the wall that is glued to the drywall at an angle, it's pretty easy. But if you try to pull the tube off that is sandwiched, you can't. You could hang your full weight on it. If you COULD pull it off, you wouldn't have a 6" hole. You'd have a hole more like 12" or 18". ALL of that surface area is involved. 

Plus, welding to the sheet metal pan could easily blow through the sheet metal, and compromise it. It's not easy to weld 2 pieces of metal together cleanly that have large differences in thickness. 

Pardon the crude example- it's how a carpenter thinks!

I remember the plates recommended to be different sizes from the off-road community and remember reading it from the late 80’s or early 90’s.

I never got proof that it can happen, just guys smarter and more experienced than me telling me it’s true! Never saw it happen though so who knows?

In reply to exST165 :

Bolting is pretty much impossible on the riser, generally there is no access vto the underside.  My cages will generally be tied into as many sections of the car as possible, with connection to the sill as much as I can.  My mounting plates are seldom minimum size or flat, or even just one piece of material.

This one will mount to the riser and seatbelt towers. Probably the strongest part of the car. 

Paul: that makes a whole lot of sense.  I can see what you mean in my head. 

Thanks y'all for the feedback 

Maybe some ideas from this removable NC Miata bar from Flyin'Miata.........

https://www.flyinmiata.com/nc-transformer-roll-bar.html

In reply to Dusterbd13 :

Your welcome.  Now back to our regularly scheduled discussion.